Beam filter assembly and beam filter positioning device

ABSTRACT

In a beam filter assembly, a base filter is employed to modify a beam quality of a radiation beam of a base energy level and a first filter slice is stacked with the base filter to modify a beam quality of a radiation beam of a first energy level higher than the base energy level. In a beam filter positioning device, a base stage carries a base filter and a first stage carries a filter slice. The base stage is provided with a first engagement site and a second engagement site. The first stage is provided with a first engagement site, a second engagement site, and an open port. The first stage and the base stage are each independently movable relative to the beamline. The first stage is engageable with the base stage when at least one of the first and second engagement sites of the first stage is aligned with at least one of the first and second engagement sites of the base stage, and is further movable with the base stage in unison when engaged.

TECHNICAL FIELD

Embodiments of this disclosure relate generally to radiation apparatusesand methods. In particular, various embodiments of a beam filterassembly and a beam filter positioning device are described.

BACKGROUND

Linear accelerators (LINACs) are used to produce high energy radiationbeams useful in radiation therapy and imaging of patients and in otherapplications such as safety inspection. A linear accelerator includesvarious devices operable to produce, condition, or monitor radiationbeams. By way of example, an x-ray target produces x-rays uponimpingement by energetic electrons. A beam filter such as a photonflattening filter conditions x-rays to provide a desired energy or dosedistribution profile across the radiation field. In an electron modeoperation of a linear accelerator, a scattering foil and button is usedto scatter incident electrons to provide a broadened, uniform beamprofile.

In conventional linear accelerators, full sized beam filters are used infiltration of x-ray beams of different energy levels. For x-ray beams ofhigh energy levels such as 18 MV, 20 MV or higher, a beam filter may becomposed of multiple materials to create a desired filtration. The useof full sized beam filters for radiation beams of different energylevels requires greater filter material volumes and sizes and increasesmanufacturing costs.

Conventional beam filter positioning devices include exchangers whichare designed to carry full sized beam filters and allow switching ofindividual filters for beams of different energy levels. For example, aconventional carousel exchanger is designed to carry five full sizedindividual filters for filtration of beams of five different energylevels. This kind of positioning device design requires more space orclearance for full sized filters' movement and more complicated motionaxes for positioning and retracting beam filters in and/or out of thebeam path.

SUMMARY

The present disclosure uses layers of filter slices and stacks themtogether to provide desired filtration. For example, a filter assemblymay include a base filter and one or more filter slices. Each of thebase filter and filter slices can be made of an x-ray attenuatingmaterial to remove energy from an x-ray beam. The beam exiting the basefilter and filter slice(s) can thus be flattened to its desired energylevel. The base filter can be configured to provide flattening for thelowest energy radiation beam output. One or more filter slices can bestacked on top or bottom of the base filter. The combination of the basefilter and a stacked filter slice can flatten an x-ray output beam at ahigher energy level than the base filter does. Multiple slices can bestacked with the base filter for filtration of an x-ray beam at a higherenergy level.

In an embodiment, a beam filter assembly includes a base filter and afirst filter slice. The base filter is configured to modify a beamquality of a radiation beam of a base energy level. The first filterslice is stackable with the base filter when in use to modify a beamquality of a radiation beam of a first energy level higher than the baseenergy level. The base filter and the first filter slice each can beindependently moved in and/or out of a beamline by a linear motion axisor a rotary motion axis. The base filter can be configured to modify anenergy distribution of the radiation beam of the base energy level, andthe first filter slice can be stacked with the base filter to modify anenergy distribution of the radiation beam of the first energy level.Alternatively or additionally, the base filter can be configured tomodify a dose distribution of the radiation beam of the base energylevel, and the first filter slices can be stacked with the base filterto modify a dose distribution of the radiation beam of the first energylevel.

In some embodiments, the beam filter assembly may further include asecond filter slice stackable with the first filter slice and/or thebase filter when in use. The second filter slice can be configured tostack with the first filter slice and the base filter when in use tomodify the beam quality of the radiation beam of the second energylevel. Alternatively, the second filter slice can be configured to stackwith the base filter when in use to modify the beam quality of theradiation beam of the second energy level. The second filter slice canbe configured to stack with the first filter slice and/or the basefilter to modify an energy distribution and/or a dose distribution ofthe radiation beam of the second energy level. The base filter, thefirst and second filter slices each can be independently moved in and/orout of a beamline.

In some embodiments, the beam filter assembly may further include athird filter slice stackable with the second and first filter slices,and/or with the base filter when in use to modify a beam quality of aradiation beam of a third energy level higher than the second energylevel. The third filter slice can be configured to stack with the secondand first filter slices, and the base filter when in use to modify thebeam quality of the radiation beam of the third energy level.Alternatively, the third filter slice can be configured to stack withthe base filter when in use to modify the beam quality of the radiationbeam of the third energy level. The third filter slice can be configuredto stack with the second and first filter slices, and/or the base filterto modify an energy distribution and/or a dose distribution of theradiation beam of the third energy level. The base filter, the first,second and third filter slices can be each independently moved in and/orout of the beamline.

In some embodiments, the beam filter assembly may further include afourth filter slice stackable with the third, second, and first filterslices, and/or with the base filter to modify a beam quality of aradiation beam of a fourth energy level higher than the third energylevel. The fourth filter slice can be configured to stack with thethird, second and first filter slices, and the base filter when in useto modify the beam quality of the radiation beam of the fourth energylevel. Alternatively, the fourth filter slice can be configured to stackwith the base filter when in use to modify the beam quality of theradiation beam of the fourth energy level. The fourth filter slice canbe configured to stack with the third, second and first filter slices,and/or the base filter to modify an energy distribution and/or a dosedistribution of the radiation beam of the fourth energy level. The basefilter, the first, second, third, and fourth filter slices can be eachindependently moved in and/or out of the beamline.

In a specific embodiment, an exemplary beam filter assembly includes abase filter configured to modify a beam quality of a radiation beam of abase energy level at about 4 MV, a first filter slice configured tostack with the base filter when in use to modify a beam quality of aradiation beam of a first energy level at about 6 MV, a second filterslice configured to stack with the first filter slice and the basefilter when in use to modify a beam quality of a radiation beam of asecond energy level at about 10 MV, a third filter slice configured tostack with the second and first filter slices and the base filter whenin use to modify a beam quality of a radiation beam of a third energylevel at about 15 MV, and a fourth filter slice configured to stack withthe third, second, and first filter slices and the base filter when inuse to modify a beam quality of a radiation beam of a fourth energylevel at about 18 MV.

In some embodiments, the disclosure provides a beam filter assemblycomprising a base filter configured to modify a beam quality of aradiation beam of a base energy level and N filter slices each beingstackable with the base filter (N being an integer of 1-10). The Nfilter slices are configured such that a 1^(st) filter slice is for useat least in modifying a radiation beam of a 1^(st) energy level, a2^(nd) filter slice for use at least in modifying a radiation beam of a2^(nd) energy level, and in such successive order that an (N−1)^(th)filter slice is for use at least in modifying a radiation beam of an(N−1)^(th) energy level, and an N^(th) filter slice for use at least inmodifying a radiation beam of an N^(th) energy level, and that theN^(th) energy level is higher than the (N−1)^(th) energy level, and insuch successive order that the 2^(nd) energy level is higher than the1^(st) energy level, and the 1^(st) energy level is higher than the baseenergy level. When an M^(th) filter slice (M being an integer ≦N) isstacked with the base filter in use to modify a radiation beam of anM^(th) energy level, the 1^(st) through (M−1)^(th) filter slices arestacked between the M^(th) filter slice and the base filter. The basefilter and the N filter slices can be each configured to modify anenergy spectrum and/or a dose distribution of a radiation beam. The basefilter and N filter slices each can be independently moved in and/or outof the beamline by a linear motion axis or a rotary motion axis.

The present disclosure further provides a beam filter positioning devicethat can index layers of filter slices. An exemplary beam filterpositioning device uses tiered linear stages to provide a desired x-rayfilter combination. Each stage may carry a filter or filter slice, beprovided with an open port, and include engagement sites for engagingwith an actuator key. Each stage may be moved to position a filter orfilter slice or an open port in the beam path. The position of the stagemay be set by an actuator key that engages the stages at one of theengagement sites to create the filter and open port combination.Scattering foils and electron buttons can also be carried by the beamfilter positioning device and indexed in a similar fashion.

In an embodiment, a beam filter positioning device includes a base stagecarrying a base filter and one or more additional stages each carries afilter slice. The base stage is provided with a first engagement siteand a second engagement site. The one or more additional stages includeat least a first stage carrying a first filter slice and being providedwith a first engagement site, a second engagement site, and an openport. The first stage and the base stage are each independently movablerelative to the beamline. The first stage is engageable with the basestage when at least one of the first and second engagement sites of thefirst stage is aligned with at least one of the first and secondengagement sites of the base stage, and further movable with the basestage in unison when engaged.

The first stage and the base stage each can be independently moved by alinear motion axis. When engaged, the first stage and the base stage canbe further moved in unison by an additional linear motion axis.Alternatively, the first stage and the base stage each can beindependently moved by a rotary motion axis, and when engaged the firststage and the base stage can be further moved in unison.

The first stage and the base stage can be arranged such that when thefirst engagement site of the first stage is aligned with the firstengagement site of the base stage, the first filter slice carried by thefirst stage is stacked with the base filter carried by the base stage,and the first stage can be engaged and further moved with the base stagein unison to position the stacked first filter slice and base filter inthe beamline; or when the second engagement site of the first stage isaligned with the first engagement site of the base stage, the open portprovided in the first stage is aligned with the base filter carried bythe base stage, and the first stage can be further moved with the basestage in unison to position the open port of the first stage and thebase filter carried by the base stage in the beamline.

In some embodiments, the first stage and the base stage each can furthercarry a scattering foil, and the first stage and the base stage can bearranged such that when the second engagement site of the first stage isaligned with the first engagement site of the base stage, the firststage can be engaged and further moved with the base stage in unison toposition the scattering foil carried by the base stage in the beamline;or when the first engagement site of the first stage is aligned with thesecond engagement site of the base stage, and the first stage can beengaged and further moved with the base stage in unison to position thescattering foil carried by the first stage in the beamline.

In some embodiments, the first stage and the base stage can be providedwith apertures at the first and second engagement sites of the firststage and base stage respectively. The beam filter positioning devicemay further include an actuator key operable to engage each of theapertures at the first and second engagement sites of the first stageand the base stage. The actuator key can be controlled by a control.

In some embodiments, the beam filter positioning device may furtherinclude a second stage carrying a second filter slice. The second stagemay be provided with a first engagement site, a second engagement site,and an open port, and independently movable relative to the beamline.The second stage is engageable with the first stage when at least one ofthe first and second engagement sites of the second stage is alignedwith at least one of the first and second engagement sites of the firststage, and further movable with the first stage in unison when engaged.The second stage is engageable with the first stage and the base stagewhen at least one of the first and second engagement sites of the secondstage is aligned with at least one of the first and second engagementsites of the first stage and the base stage, and further movable withthe first stage and the base stage in unison when engaged. The firststage, the second stage, and the base stage each can be independentlymoved by a linear motion axis. When engaged the first stage and thesecond stage can be further moved in unison by an additional linearmotion axis. Further, when engaged the first stage, the second stage andthe base stage can be further moved in unison.

The first stage, the second stage, and the base stage each can beprovided with apertures at the first and second engagement sites of thefirst stage, the second stage, the base stage respectively, and the beamfilter positioning device may further include an actuator key operableto engage each of the apertures at the first and second engagement sitesof the first stage, the second stage, and the base stage respectively.The actuator key can be controlled by a control.

The first stage, the second stage, and the base stage can be arrangedsuch that when the first engagement sites of the first and second stagesare aligned with the first engagement site of the base stage, the firstfilter slice carried by the first stage and the second filter slicecarried by the second stage are stacked with the base filter carried bythe base stage, and the first and second stages can be engaged andfurther moved with the base stage in unison to position stacked firstand second filter slices and base filter in the beamline; or when thefirst engagement sites of the first and second stages are aligned withthe second engagement site of the base stage, the open ports in thefirst and second stages are aligned with the base filter carried by thebase stage, and the first and second stages can be engaged and furthermoved with the base stage in unison to position the aligned open portsand base filter in the beamline.

The first stage, the second stage, and the base stage each can furthercarry a scattering foil, and the first stage, the second stage, and thebase stage can be arranged such that when the second engagement sites ofthe first and second stages are aligned with the first engagement siteof the base stage, the first and second stages can be engaged andfurther moved with the base stage in unison to position the scatteringfoil carried by the base stage in the beamline; or when the firstengagement site of the first stage is aligned with the second engagementsites of the second stage and base stage, and the first stage can beengaged and further moved with the second stage and the base stage inunison to position the scattering foil carried by the first stage in thebeamline; or when the first engagement site of the second stage isaligned with the second engagement sites of the first stage and basestage, and the second stage can be engaged and further moved with thefirst stage and the base stage in unison to position the scattering foilcarried by the second stage in the beamline.

In some embodiments, the beam filter positioning device can include 1-10additional stages each carrying a filter slice, provided with an openport, a first engagement site and a second engagement site, andindependently movable relative to the beamline. The base stage and eachof the 1-10 additional stages may further carry a scattering foil. Thebase stage and each of 1-10 additional stages each can be independentlymoved by a linear motion axis. The base stage can be engaged and movedwith the 1-10 additional stages in unison by an additional linear motionaxis.

This Summary is provided to introduce selected embodiments in asimplified form and is not intended to identify key features oressential characteristics of the claimed subject matter, nor is itintended to be used as an aid in determining the scope of the claimedsubject matter. The selected embodiments are presented merely to providethe reader with a brief summary of certain forms the invention mighttake and are not intended to limit the scope of the invention. Otheraspects and embodiments of the disclosure are described in the sectionof Detailed Description.

BRIEF DESCRIPTION OF THE DRAWINGS

These and various other features and advantages will become betterunderstood upon reading of the following detailed description inconjunction with the accompanying drawings and the appended claimsprovided below, where:

FIG. 1 schematically shows an exemplary beam filter assembly accordingto embodiments of the disclosure;

FIGS. 2A through 2E schematically show various combinations of filterslices with a base filter of an exemplary beam filter assembly accordingto embodiments of the disclosure;

FIG. 3 is a cutaway view of an exemplary radiation apparatus including abeam filter positioning device according to embodiments of thedisclosure;

FIG. 4 is an iso bottom view of a beam filter positioning deviceaccording to embodiments of the disclosure;

FIGS. 5A and 5B schematically show two movable stages each carrying abeam filter or filter slice and a scattering foil according toembodiments of the disclosure;

FIGS. 6A and 6B schematically show positioning of a base filter in thebeamline of a radiation apparatus according to embodiments of thedisclosure;

FIGS. 7A and 7B schematically show stacking and positioning of a basefilter and a filter slice in the beamline of a radiation apparatusaccording to embodiments of the disclosure;

FIGS. 8A and 8B schematically show positioning of a scattering foil inthe beamline of a radiation apparatus according to embodiments of thedisclosure; and

FIGS. 9A through 9D schematically show positioning of another scatteringfoil in the beamline of a radiation apparatus according to embodimentsof the disclosure.

DETAILED DESCRIPTION

Various embodiments of beam filter assemblies and beam filterpositioning devices are described. It is to be understood that thedisclosure is not limited to the particular embodiments described. Anaspect described in conjunction with a particular embodiment is notnecessarily limited to that embodiment and can be practiced in any otherembodiments.

Various embodiments are described with reference to the figures. Itshould be noted that some figures are not necessarily drawn to scale.The figures are only intended to facilitate the description of specificembodiments, and are not intended as an exhaustive description or as alimitation on the scope of the disclosure. Further, in the figures anddescription, specific details may be set forth in order to provide athorough understanding of the disclosure. It will be apparent to one ofordinary skill in the art that some of these specific details may not beemployed to practice embodiments of the disclosure. In other instances,well known components or process steps may not be shown or described indetail in order to avoid unnecessarily obscuring embodiments of thedisclosure.

All technical and scientific terms used herein have the meaning ascommonly understood by one of ordinary skill in the art unlessspecifically defined otherwise. As used in the description and appendedclaims, the singular forms of “a,” “an,” and “the” include pluralreferences unless the context clearly dictates otherwise. The term “or”refers to a nonexclusive “or” unless the context clearly dictatesotherwise. Further, the term “first” or “second” etc. may be used todistinguish one element from another in describing various elements e.g.two or more than two elements. It should be noted the terms “first” and“second” as used herein include references to two or more than two. Forexample, a beam filter position device may include two or more movablestages each carrying a filter slice. Further, the use of the term“first” or “second” should not be construed as in any particular orderunless the context clearly dictates otherwise.

Various relative terms such as “top,” “bottom,” “above,” “under,”“upper,” and “lower,” etc. may be used to facilitate description ofvarious embodiments. The relative terms are defined with respect to aconventional orientation of a structure and do not necessarily representan actual orientation of the structure in manufacture or use. Thefollowing detailed description is, therefore, not to be taken in alimiting sense.

As used herein, the phrase “a radiation beam of an energy level” refersto a beam output of a radiation machine operated at an energy level. Byway of example, a radiation beam of an energy level at 4 MV refers to abeam produced by a radiation machine operated at the energy level of 4MV. Similarly, a radiation beam of an energy level at 18 MV refers to abeam produced by a radiation machine operated at the energy level of 18MV, and so on.

As used herein, the phrase “a beam quality of a radiation beam” refersto an energy spectrum of a radiation beam, a dose distribution of aradiation beam, or the shape or direction of a radiation beam. Aradiation beam includes but is not limited to a beam of x-rays, protons,heavy ions, electrons or the like.

As used herein, the term “beam filter” and “base filter” may be usedinterchangeably and refer to a component that can be used independentlyor in combination with one or more filter slices to modify a beamquality of a radiation beam of an energy level useful in clinicalapplications. By way of example, a beam filter may be designed,engineered or configured to remove the lower energy from a radiationbeam, to flatten a dose distribution of a radiation beam, and/or toimprove the symmetry of a radiation beam.

As used herein, the term “filter slice” refers to a component that isused in combination with a base filter or with a base filter and otherfilter slice(s) to collectively modify a beam quality of a radiationbeam of an energy level. In general, a filter slice is not usedindependently or with other filter slices only in clinical applications.

As used herein, the term “scattering foil” refers to a component that isused to broaden the profile of a thin beam of electrons, or protons,etc.

Layering of Filters for Radiation Beam Filtration

FIG. 1 schematically shows an exemplary beam filter assembly 100according to embodiments of the disclosure. The beam filter assembly 100includes a base filter 102 and a plurality of filter slices 104 a-104 dstackable with the base filter 102. Four filter slices are shown in FIG.1 for illustration purpose. It should be appreciated that fewer or morethan four filter slices can be included in the beam filter assembly ofthe disclosure. For example, the beam filter assembly of the disclosurecan include a base filter and a single filter slice. The beam filterassembly may also include a base filter and multiple filter slices e.g.up to 10, 20, 50, or more. In general, the beam filter assembly of thedisclosure may include as many filter slices as the number of the energylevels under which a radiation machine operates. Further, in FIG. 1 thefilter slices 104 a-104 d are shown as being stacked on top of the basefilter 102. It should be appreciated that the beam filter assembly 100can be arranged or configured such that the filter slices 104 a-104 dare stackable under the base filter 102 when in use.

The base filter 102 can be designed to modify the characteristics of aradiation beam of the lowest energy level under which a radiationmachine operates. For ease of description, the letter “X” is used hereinto refer to the energy level (MV) under which a radiation machineoperates. By way of example, the term “4X base filter” refers to a basefilter that is configured to modify a beam quality of a radiation beamproduced by a radiation machine operated at the energy level of 4 MV.The base filter can be any of 1X to 18X filter.

The base filter 102 can be used independently in clinical or otherapplications. In various embodiments of the disclosure, the base filter102 is used in combination with one or more filter slices 104 a-104 d tomodify beam qualities of radiation beams of higher energy levels, aswill be described in greater detail below.

The base filter 102 may be configured to modify the energy spectrum,dose distribution, or intensity of a radiation beam, etc. An unfilteredradiation beam produced by a megavoltage radiation machine typically hasa sharply peaked distribution with respect to energy, dose rate, orintensity etc. Therefore, beam filters are typically used to conditionor modify the characteristics of the beams in order to meet therequirements for clinical applications. By way of example, a beam filtercan be designed to selectively remove low energy from the energyspectrum of a beam to provide a “hardened” high energy beam. In anotherexample, a beam filter can be used to flatten the dose distributioncurve of a radiation beam by e.g. reducing the dose rate at the beamcenter. The symmetry of a radiation beam can also be improved by using abeam filter. In general, a beam filter can be made of a partiallyradiation attenuating material. Suitable materials include but are notlimited to copper, aluminum, lead, or the like. Manufacturing of variousbeam filters is known in the art and its detailed description istherefore omitted herein. In some embodiments, a beam filter can be madeof multiple materials to create a desired filtration.

Referring to FIG. 1, the filter slice(s) 104 a-104 d is (are) stackablewith the base filter 102. In use, the filter slice(s) 104 a-104 d is(are) stacked or combined with the base filter 102 to collectivelymodify the characteristics of a beam of an energy level higher than thatof the beam modified by the base filter 102 alone. FIG. 1 shows anembodiment where multiple filter slices 104 a-104 d are stacked with thebase filter 102 in use. In some embodiments, each of the multiple filterslices 104 a-104 d can be designed to separately or independently stackwith the base filter 102 for modifying a radiation beam of a higherenergy level. By way of example, a base filter 102 can be a 4X basefilter. A first filter slice 104 a can be designed to stack with thebase filter 102 and the stacked first filter slice 104 a/base filter 102modifies a radiation beam of an energy level at 6 MV when in use. Inanother example, a second filter slice 104 b can be designed to stackwith the base filter 102 and the stacked second filter slice 104 b/basefilter 102 modifies a radiation beam of an energy level at 10 MV when inuse. In a further example, a third filter slice 104 c can be designed tostack with the base filter 102 and the stacked third filter slice 104c/base filter 102 modifies a radiation beam of an energy level at 15 MVwhen in use. In a further example, a fourth filter slice 104 d isdesigned to stack with the base filter 102 and the stacked fourth filterslice 104 d/base filter 102 modifies a radiation beam of an energy levelat 18 MV when in use, and so on.

The above examples are provided for illustration purpose. It should beappreciated that the base filter 102 can be designed to modify aradiation beam of an energy level lower or greater than 4 MV. Forexample, the base filter 102 can be a 1X filter, 2X filter, 3X filter,5X filter, or 6X filter, etc., and each of the filter slices 104 a-104 dcan be configured such that the stacked base filter and one of thefilter slices 104 a-104 d can modify a beam quality of a radiation beamof a higher energy level.

The base filter 102 and filter slice(s) 104 a-104 d can be stacked usingmovable stages, as will be described in greater detailed below. Each ofthe movable stages can be driven by a motion axis such as a linear orrotary motion axis to allow stacking and positioning of the stackedfilter slice/base filter in and/or out of the beam path. Feedbacksensors can be used to confirm alignment or positions of the base filter102 and filter slices 104 a-104 d. Feedback sensors can be provided onthe filter and/or filter slices, movable stages, or any other suitablelocations providing signals capable of determining positions of thefilter and/or filter slices. In some embodiments, an ion chamber can beused as a secondary check for beam quality to confirm the beam profile,flatness, or symmetry, etc.

In various embodiments of the disclosure, a plurality of filter slicescan be stacked with a base filter when in use, as shown in FIG. 1. Insuch embodiments, filter slices are designed such that they can beshared between energy levels when in use. Combinations of filter slicescan be arranged according to the energy levels of radiation beams to bemodified, as will be described in greater detail below in connectionwith FIGS. 2A-2E.

FIGS. 2A through 2E illustrate various combinations or arrangements offilter slices with a base filter of a beam filter assembly according toembodiments of the disclosure. The beam filter assembly 200 includes abase filter 202, a first filter slice 204 a, a second filter slice 204b, a third filter slice 204 c, and a fourth filter slice 204 d. Itshould be noted that the principle described herein can be applied to abeam filter assembly that includes fewer or more than four filterslices.

FIG. 2A shows a base filter 202 (e.g. 4X base filter) positioned in aradiation beam. In this arrangement of the beam filter assembly 200, thebase filter 102 is used alone or independently in modifying a radiationbeam of a base energy level at 4 MV. In this arrangement, the firstthrough fourth filter slices 204 a-204 d (not shown in FIG. 2A) areretracted from the beam path such that they are not stacked with thebase filter 202.

FIG. 2B shows an arrangement of the beam filter assembly 200 where the4X base filter 202 and the first filter slice 204 a are stacked andpositioned in a beam path. The first filter slice 204 a can be designedsuch that the combination of the first filter slice 204 a and the basefilter 202 modifies a radiation beam of an energy level at e.g. 6 MV.For purpose of description, the first filter slice 204 a is hereafterreferred to as (6X-4X) delta slice. In this arrangement, the secondthrough fourth filter slices 204 b-204 d (not shown in FIG. 2B) areretracted from the beam path such that they are not stacked with thebase filter 202 and the first filter slice 204 a. The arrangementsillustrated in FIGS. 2A and 2B show that the base filter (4X) of thebeam filter assembly 200 can be used or shared in modifying radiationbeams of energy levels at both 4 MV and 6 MV.

FIG. 2C shows an arrangement of the beam filter assembly 200 where the4X base filter 202, the first filter slice 204 a ((6X-4X) delta slice),and the second filter slice 204 b are stacked and positioned in a beampath. The second filter slice 204 b can be designed such that itscombination with the first filter slice 204 a ((6X-4X) delta slice) andthe 4X base filter 202 modifies a radiation beam of an energy level e.g.at 10 MV. For purpose of description, the second filter slice 204 b ishereafter referred to as (10X-6X) delta slice. In this arrangement, thethird and fourth filter slices 204 c-204 d (not shown in FIG. 2C) areretracted such that they are not stacked with the second filter slice204 b, the first filter slice 204 a, and the base filter 202. Thearrangements illustrated in FIGS. 2A, 2B and 2C show that the basefilter (4X) of the beam filter assembly 200 can be used or shared inmodifying radiation beams of energy levels at all of 4 MV, 6 MV, and 10MV. The first filter slice 204 a, or (6X-4X) delta slice, of the beamfilter assembly 200 can be used or shared in modifying radiation beamsof energy levels at both 6 MV and 10 MV.

FIG. 2D shows an arrangement of the beam filter assembly 200 where the4X base filter 202, the first filter slice 204 a ((6X-4X) delta slice),the second filter slice 204 b ((10X-6X) delta slice), and the thirdfilter slice 204 c are stacked and positioned in a beam path. The thirdfilter slice 204 c can be designed such that its combination with thesecond filter slice 204 b ((10X-6X) delta slice), the first filter slice204 a ((6X-4X) delta slice), and the 4X base filter 202 modifies aradiation beam of an energy level e.g. at 15 MV. For purpose ofdescription, the third filter slice 204 c is referred to as (15X-10X)delta slice. In this arrangement, the fourth filter slice 204 d (notshown in FIG. 2D) is retracted such that it is not stacked with thefirst through third filter slices 204 a-204 c and the base filter 202.The arrangements illustrated in FIGS. 2A, 2B, 2C, and 2D show that thebase filter (4X) of the beam filter assembly 200 can be used or sharedin modifying radiation beams of energy levels at all of 4 MV, 6 MV, 10MV, and 15 MV. The first filter slice 204 a, or (6X-4X) delta slice ofthe beam filter assembly 200, can be used or shared in modifyingradiation beams of energy levels at all of 6 MV, 10 MV, and 15 MV. Thesecond filter slice 204 b, or (10X-6X) delta slice, can be used orshared in modifying radiation beams of energy levels at both 10 MV and15 MV.

FIG. 2E shows an arrangement of the beam filter assembly 200 where the4X base filter 202, the first filter slice 204 a ((6X-4X) delta slice),the second filter slice 204 b ((10X-6X) delta slice), the third filterslice 204 c ((15X-10X) delta slice), and the fourth filter slice 204 dare stacked and positioned in a beam path. The fourth filter slice 204 dcan be designed such that its combination with the third, second, andfirst filter slices 204 a-204 c and the base filter 202 modifies aradiation beam of an energy level e.g. at 18 MV. For purpose ofdescription, the fourth filter slice 204 d is hereafter referred to as(18X-15X) delta slice. The arrangements illustrated in FIGS. 2A, 2B, 2C,2D, and 2E show that the base filter (4X) of the beam filter assembly200 can be used or shared in modifying radiation beams of energy levelsat all of 4 MV, 6 MV, 10 MV, 15 MV, and 18 MV. The first filter slice204 a, or (6X-4X) delta slice, can be used or shared in modifyingradiation beams of energy levels at all of 6 MV, 10 MV, 15 MV and 18 MV.The second filter slice 204 b, or (10X-6X) delta slice, can be used orshared in modifying radiation beams of energy levels at all of 10 MV, 15MV, and 18 MV. The third filter slice 204 c, or (18X-10X) delta slice,can be used or shared in modifying radiation beams of energy levels atboth 15 MV and 18 MV.

In conventional linear accelerators, full sized filters are used infiltration of radiation beams of different energy levels. For example,to modify radiation beams of five different energy levels at 4 MV, 6 MV,10 MV, 15 MV and 18 MV respectively, five different individual beamfilters are used, each being a full sized filter and used independently.This conventional design requires a greater total amount of attenuation,a greater material volume and size, thus increases the manufacturingcosts. Further, a device for exchanging and storing full sized beamfilters requires more clearance and takes up more space inside theradiation machine. Motion axes for positioning beam filters in and/orout of the beam path are also more complicated. One of the advantages ofthe beam filter assembly of this disclosure is layering of beam filtersby using filter slices which can be stacked with a base filter toprovide various combinations for filtration of radiation beams ofdifferent energy levels. Filter slices can be used or shared betweendifferent energy levels. As a result, the total amount of attenuation,volume, and size of the beam filter assembly can be reduced, leading tosignificant reduction of manufacturing costs. The motion axes forpositioning and retracting filter slices can also be simplified, as willbe described in greater detail below.

Indexing of Layered Beam Filters

FIG. 3 schematically shows a radiation apparatus 300 according toembodiments of the disclosure. FIG. 3 is a cut-away view showingcomponents of the radiation apparatus 300 in cross-sections. In general,the radiation apparatus 300 includes a source (not shown) and a beamfilter positioning device 302 supported by a frame structure 301. Thesource is operable to produce a radiation beam. The beam filterpositioning device 302 operates to position a beam filter and/or othercomponents relative to the source. The source may be a source ofelectrons or a source of x-rays which may be generated upon impingementof a target by electrons. The source may also be a source of protons orheavy ions. The beam of electrons or x-rays has a central beam path orbeamline indicated at 304. The beamline 304 is typically fixed relativeto the frame structure 301. The beam filter positioning device 302 ismovable relative to the frame structure 301, e.g. in y-direction asshown, to position a beam filter or other components in and/or out ofthe beamline 304.

The beam filter positioning device 302 may include a movable stage 306(hereafter “Y-stage”). The movable Y-stage 306 allows components ordevices supported thereby to move along y-direction, towards or awayfrom the beamline 304. For example, the Y-stage may support, move andposition a collimator 307 in the beamline 304 to generally define thesize and shape of the x-ray field, or move the collimator 307 away fromthe beamline 304 to allow other components such as a scattering foil inthe path of electron beams. Shielding 308, 309 disposed adjacent to thecollimator 307 may also mount to the Y-stage 306. In various embodimentsof the disclosure, the Y-stage 306 may carry a plurality of plates orstages 310 a-310 e, which may respectively carry a base filter or filterslice, as will be described in greater detail below. The Y-stage 306 maybe driven by a motion axis (not shown) secured to the frame structure301.

Still referring to FIG. 3, the beam filter positioning device 302 mayinclude an actuator key 312 operable to engage the stages 310 a-310 e.The actuator key 312 may be actuated either by a linear axis or by arotary axis (not shown) to allow a shaft to protrude into or pull out ofengagement apertures in the stages 310 a-310 e, as will be described ingreater detailed below. The actuator key 312 may mount to the Y-stage306, allowing the engaged stages to further move in unison with theY-stage 306.

FIG. 4 is an iso bottom view of a beam filter positioning device 302according to embodiments of the disclosure. As shown, the beam filterpositioning device 302 includes a plurality of stages 310 a-310 e, whichmay be supported by blocks 314 a, 314 b. Each of the stages 310 a-310 eis independently movable relative to the blocks 314 a, 314 b, e.g.,driven by a linear motion axis (not shown). By way of example, each ofthe stages 310 a-310 e, e.g. 310 a may be provided with slide membere.g. 316 a, 316 b on either side of the stage, allowing the stage tomove along corresponding guide rail members e.g. 318 a, 318 b providedin the blocks 314 a, 314 b. The guide rail members 318 a, 318 b in theblocks 314 a, 314 b allow the stage 310 a to move in the longitudinaldirection and hold the stage in the lateral and elevational directions.Any mechanisms that allow each of the stages 310 a-310 e toindependently move may be used. In alternative embodiments, each of theplurality of stages 310 a-310 e may be moved by a rotary motion axis.The blocks 314 a, 314 b may mount to and move with the Y-stage 306.Therefore, in addition to the independent movement relative to theblocks 314 a, 314 b, the stages 310 a-310 e may further move with theY-stage 306.

For purpose of description, the bottom stage 310 a shown in FIG. 4 maybe hereafter referred to as a base stage which may carry a base filter.The other stages tiered above the bottom stage 310 a may be referred toas slice stages 310 b-310 e, each of which may carry a filter slice. Itshould be noted that alternatively the top stage may be used to carry abase filter and the other stages tiered below the top stage may be usedto carry filter slices. Further, while five stages are shown in FIG. 4,fewer or more than five stages may alternatively be used. For example,the beam filter positioning device 302 may include a base stage and oneslice stage. Alternatively, the beam filter positioning device 302 mayinclude a base stage and up to 10 or more slice stages to providevarious combinations of filter layering. The present disclosure is notlimited to the number of the individual stages.

FIGS. 5A and 5B schematically show an exemplary base stage 310 a and anexemplary slice stage 310 b. In FIGS. 5A-5B, the base stage 310 a andslice stage 310 b are shown spaced apart in order to illustrate featuresof the stages and components carried by the stages with greater clarity.In a beam filter positioning device, the stages 310 a, 310 b can bearranged closely next to each other. As shown, the base stage 310 acarries a base filter 320 a. The slice stage 310 b carries a filterslice 320 b. Additionally or alternatively, the base stage 310 a maycarry a scattering foil 324 a and be provided with an open port 326 a,the slice stage 310 b may carry a scattering foil 324 b and be providedwith an open port 326 b. The base stage 310 a has a first engagementsite 328 a and a second engagement site 330 a. The slice stage 310 b hasa first engagement site 328 b and a second engagement site 330 b. At thefirst and second engagement sites 328 a, 330 a of the base stage 310 a,apertures may be provided for receiving the actuator key 312. At thefirst and second engagement sites 328 b, 330 b of the slice stage 310 b,apertures may be provided for receiving the actuator key 312. While notshown in FIGS. 5A-5B, reference 328 c-328 e may be used hereafter todescribe the first engagements sites in the slice stages 310 c-310 erespectively, and reference 330 c-330 e may be used hereafter todescribe the second engagements sites in the slice stages 310 c-310 erespectively. Apertures may be provided at each of the first engagementssites 328 c-328 e and second engagement sites 310 c-310 e for receivingthe actuator key 312.

Referring to FIG. 5A, the base stage 310 a and/or the slice stage 310 bmay be positioned such that the first engagement site 328 a of the basestage 310 a is aligned with the first engagement site 328 b of the slicestage 310 b. The alignment shown in FIG. 5A allows the filter slice 320b on the slice stage 310 b to stack with the base filter 320 a on thebase stage 310 a. The actuator key 312 may be actuated to engage theslice stage 310 b and base stage 310 a at the first engagement sites 328a, 328 b. The engaged slice stage 310 b/base stage 310 a may be thenmoved in unison to position the stacked filter slice 320 b/base filter320 a in a beam path. FIG. 5A shows an exemplary actuator key 312,including a body 332 and an elongate shaft 334 supported by the body332. A plunger structure 336, which may be driven either linearly orrotationally, moves the body 332 and shaft 334 up and down, causing theshaft 334 to protrude into or pull out of the apertures at the firstengagement sites 328 a, 328 b, engaging the slice stage 310 b with thebase stage 310 a, or disengaging the slice stage 310 b from the basestage 310 a. The actuator key 312 may mount to the Y-stage 306 e.g. viaa handle 338, allowing the engaged base stage 310 a and slice stage 310b to move in unison toward or away from the beam path.

Referring to FIG. 5B, the base stage 310 a and/or the slice stage 310 bmay be positioned such that the second engagement site 330 b of theslice stage 310 b is aligned with the first engagement site 328 a of thebase stage 310 a. The alignment shown in FIG. 5B allows the filter slice320 b to stagger with base filter 320 a. In other words, the alignmentof the second engagement site 330 b of the slice stage 310 b with thefirst engagement site 328 a of the base stage 310 a allows the basefilter 320 a on the base stage 310 a to be exposed under the open port326 b in the slice stage 310 b. The actuator key 312 may be actuated toengage the slice stage 310 b and base stage 310 a, by causing the shaft334 to protrude into the apertures at the second engagement site 330 bof the slice stage 310 b and the first engagement site 328 a of the basestage 310 a. The engaged base stage 310 a and slice stage 310 b can bethen moved in unison to position the base filter 320 a under the openport 326 b in a beam path, or the scattering foil 324 a in a beam path.

The first engagement sites 328 a-328 e and the second engagement sites330 a-330 e on the base stage 310 a and slice stages 310 b-310 eprovides a means for indexing the base filter 320 a, slice filters 320b-320 e, scattering foils 324 a-324 e, and open ports 326 a-326 e. Theactuator key 312 operates to engage properly aligned stages, allowingselected filter, filter slice(s) or scattering foil in a beam path, aswill be described in greater detail in the following Examples.

EXAMPLES

FIGS. 6A-6B through 9A-9C illustrate various operation modes of theradiation apparatus 300 described above in connection with FIGS. 3-5.For purpose of illustration, the base stage 310 a is shown to carry abase filter (4X base filter) and a base scattering foil. The first slicestage 310 b carries a first filter slice (6X-4X delta slice) and a firstscattering foil. The second slice stage 310 c carries a second filterslice (10X-6X delta slice) and a second scattering foil. The third slicestage 310 d carries a third filter slice (15X-10X delta slice) and athird scattering foil. The fourth slice stage 310 e carries a fourthfilter slice (18X-15X delta slice) and a fourth scattering foil, and soon. It should be noted that some details such as specific base filterand filter slices for operation at particular energy levels are setforth in order to provide a thorough understanding of the disclosure. Itis apparent to one of ordinary skill in the art that these specificdetails may not be employed or required to practice embodiments of thedisclosure.

FIG. 6A-6B illustrate an operation mode of the radiation apparatus 300,in which the base filter 320 a (4X) is selected and positioned in thebeam path 304 for modifying a radiation beam of 4 MV. From a positionshown in FIG. 3, the actuator key 312 disengages the base stage 310 a atthe second engagement site 330 a but still engages the first throughfourth slice stages 310 b-310 e at their second engagement sites 330b-330 e. The engaged first through fourth slice stages 310 b-310 e movein unison with Y-stage 306 away from the beamline 304 to a locationwhere the second engagement sites 330 b-330 e of the first throughfourth slice stages 310 b-310 e align with the first engagement site 328a of the base stage 310 a, as shown in FIG. 6A. As shown in FIG. 6A, thebase filter 320 on the base stage 310 a is now aligned with the openports in the first through fourth slice stages 310 b-310 e. The actuatorkey 312 is actuated to extend further to engage the base stage 310 a atthe first engagement site 328 a. The engaged based stage 310 a and firstthrough fourth slice stages 310 b-310 e can be then moved in unison withthe Y-stage 306, towards the beamline 304, positioning or exposing thebase filter 320 a in the beamline 304, as shown in FIG. 6B. A radiationbeam of an energy level at 4 MV can be then generated and conditioned bythe 4X base filter.

FIGS. 7A-7B illustrate stacking of the first filter slice 320 b (6X-4Xdelta slice) with the base filter 320 a (4X) and positioning of thestacked first filter slice 320 b and base filter 320 a in the beamline304 for modifying a radiation beam of an energy level at 6 MV. From aposition shown in FIG. 3, the actuator key 312 disengages the base stage310 a and the first slice stage 310 b at their second engagement sites330 a, 330 b but still engages the second through fourth slice stages310 c-310 e at their second engagement sites 330 c-330 e. The engagedsecond through fourth slice stages 310 c-310 e move in unison withY-stage 306 away from the beamline 304 to a location where the secondengagement sites 330 c-330 e of the second through fourth stages 310c-310 e align with the first engagement sites 328 a, 328 b of the basestage 310 a and first slice stage 310 b as shown in FIG. 7A. As shown inFIG. 7A, the base filter 320 a and the first filter slice 320 b are nowstacked and aligned with the open ports in the second through fourthslice stages 310 c-310 e. The actuator key 312 can be actuated to extendfurther in the apertures at the first engagement sites 330 a, 330 b ofthe base stage 310 a and the first slice stage 310 b. The engaged basestage 310 a and first through fourth slice stages 310 b-310 e can bethen moved in unison with the Y-stage 306, towards the beamline 304,positioning or exposing the stacked first filter slice 320 b and thebase filter 320 a in the beamline 304, as shown in FIG. 7B. A radiationbeam of an energy level at 6 MV can be then generated and conditioned bythe stacked 6X-4X delta slice and 4X base filter.

Using the indexing and engagement methods similar to the above shown inFIGS. 7A-7B, the base filter 320 a can be stacked with the first andsecond filter slices 320 b, 320 c, and the stacked base filter 320 a andfirst and second slices 320 b, 320 c can be exposed in the beam path tocondition a radiation beam of an energy level at 10 MV. In the similarfashion, the base filter 320 a can be stacked with the first throughthird filter slices 320 b-320 d and the stacked base filter 320 a andfirst through third slices 320 b-320 d can be exposed in the beam pathto condition a radiation beam of an energy level at 15 MV. Similarly,the base filter 320 a can be stacked with the first through fourthfilter slices 320 b-320 e and the stacked base filter 320 a and firstthrough fourth slices 320 b-320 e can be exposed in the beam path tocondition a radiation beam of an energy level at 18 MV, and so on.

FIGS. 8A and 8B illustrate an operation mode of the radiation apparatus300, in which a scattering foil 324 a carried by the base stage 310 a isselected and positioned in the beamline 304 to condition an electronbeam of a base energy level. From a position shown in FIG. 3, theactuator key 312 disengages the base stage 310 a at the secondengagement site 330 a but still engages the first through fourth slicestages 310 b-310 e at their second engagement sites 330 b-330 e. Theengaged first through fourth slice stages 310 b-310 e move in unisonwith Y-stage 306 away from the beamline 304 to a location where thesecond engagement sites 330 b-330 e of the first through fourth slicestages 310 b-310 e align with the first engagement site 328 a of thebase stage 310 a as shown in FIG. 8A. A shown in FIG. 8A, the scatteringfoil 324 a staggers from the other scattering foils on the slice stages310 b-310 e. The actuator key 312 is actuated to extend further toengage the base stage 310 a at the first engagement site 328 a. Theengaged based stage 310 a and first through fourth slice stages 310b-310 e can be then moved in unison with the Y-stage 306, further awayfrom the beamline 304, positioning the scattering foils 324 a on thebase stage 310 a in the beamline 304, as shown in FIG. 8B. An electronbeam of a base energy level can then be produced and scattered by thescattering foil 324 a.

FIGS. 9A through 9D illustrate an operation mode of the radiationapparatus 300, in which the scattering foil 324 b carried by the firstslice stage 310 b is selected and positioned in the beam path tocondition an electron beam of a first energy level. From a positionshown in FIG. 3, the actuator key 312 engages the base stage 310 a andthe first through fourth slice stages 310 b-310 e at the secondengagement sites 330 a-330 e. The engaged base stage 310 a and firstthrough fourth slice stages 310 b-310 e move in unison with the Y-stage306, away from the beamline 304 as shown in FIG. 9A. The actuator key312 is then actuated to disengage the base stage 310 a and first slicestage 310 b but still engage with the second through fourth slice stages310 c-310 e. The engaged second through fourth slice stages 310 c-310 ecan be then moved in unison, further away from the beamline 304, to alocation wherein the second engagements sites 330 c-330 e of the secondthrough fourth slice stages 310 c-310 e align with the first engagementsites 328 a-328 b of the base stage 310 a and the first slice stage 310b, as shown in FIG. 9B. At this location, the scattering foil 324 a onthe base stage 310 a and first scattering foil 324 b on the first slicestage 310 a are aligned but stagger from other scattering foils on thesecond through fourth slice stages 310 c-310 e, as shown in FIG. 9B.Then, the base stage 310 a can be individually moved away from thebeamline 304 to align the second engagement site 330 a with the secondengagement sites 330 c-330 e of the second through fourth slice stages310 c-310 e and with the first engagement site 328 b of the first slicestage 310 b, as shown in FIG. 9C. The actuator key 314 is then actuatedto further engage the first engagement site 328 b of the first slicestage 310 b and the second engagement site 330 a of the base stage 310a. The engaged first through fourth slice stages 310 b-310 e and thebase stage 310 a can be then moved in unison with the Y-stage 306,further away from the beamline 304, to position the first scatteringfoil 324 b carried by the first slice stage 310 b in the beamline 304,as shown in FIG. 9D. An electron beam of a first energy level can thenbe produced and scattered by the first scattering foil 324 b.

Using the similar fashion discussed above in connection with FIGS.9A-9E, the scattering foils on the second, third or fourth slice stages310 b-310 e can be positioned in the beamline 304 respectively, tocondition electron beams of different energy levels.

Beam filter assemblies and beam filter positioning devices aredescribed. Those skilled in the art will appreciate that various othermodifications may be made within the spirit and scope of the invention.All these or other variations and modifications are contemplated by theinventors and within the scope of the invention.

1. A beam filter assembly, comprising: a base filter configured tomodify a beam quality of a radiation beam of a base energy level; and afirst filter slice, wherein the first filter slice is stackable with thebase filter when in use to modify a beam quality of a radiation beam ofa first energy level higher than the base energy level.
 2. The beamfilter assembly of claim 1, wherein the base filter and the first filterslice are each independently movable in and/or out of a beamline.
 3. Thebeam filter assembly of claim 1, wherein the base filter and the firstfilter slice are each independently movable in and/or out of a beamlineby a linear motion axis.
 4. The beam filter assembly of claim 1, whereinthe base filter and the first filter slice are each independentlymovable in and/or out of a beamline by a rotary motion axis.
 5. The beamfilter assembly of claim 1, wherein the base filter is configured tomodify an energy spectrum of the radiation beam of the base energylevel, and the first filter slices is stackable with the base filterwhen in use to modify an energy spectrum of the radiation beam of thefirst energy level.
 6. The beam filter assembly of claim 1, wherein thebase filter is configured to modify a dose distribution of the radiationbeam of the base energy level, and the first filter slices is stackablewith the base filter when in use to modify a dose distribution of theradiation beam of the first energy level.
 7. The beam filter assembly ofclaim 1, further comprising a second filter slice stackable with thefirst filter slice and/or the base filter when in use to modify a beamquality of a radiation beam of a second energy level higher than thefirst energy level.
 8. The beam filter assembly of claim 7, wherein thesecond filter slice is configured to stack with the first filter sliceand the base filter when in use to modify the beam quality of theradiation beam of the second energy level.
 9. The beam filter assemblyof claim 7, wherein the second filter slice is configured to stack withthe base filter when in use to modify the beam quality of the radiationbeam of the second energy level.
 10. The beam filter assembly of claim7, wherein the second filter slice is configured to stack with the firstfilter slice and/or the base filter when in use to modify an energyspectrum and/or a dose distribution of the radiation beam of the secondenergy level.
 11. The beam filter assembly of claim 7, wherein the basefilter, the first and second filter slices are each independentlymovable in and/or out of a beamline.
 12. The beam filter assembly ofclaim 7, further comprising a third filter slice stackable with thesecond and first filter slices, and/or with the base filter when in useto modify a beam quality of a radiation beam of a third energy levelhigher than the second energy level.
 13. The beam filter assembly ofclaim 12, wherein the third filter slice is configured to stack with thesecond and first filter slices and the base filter when in use to modifythe beam quality of the radiation beam of the third energy level. 14.The beam filter assembly of claim 12, wherein the third filter slice isconfigured to stack with the base filter when in use to modify the beamquality of the radiation beam of the third energy level.
 15. The beamfilter assembly of claim 12, wherein the third filter slice isconfigured to stack with the second and first filter slices, and/or withthe base filter when in use to modify an energy spectrum and/or a dosedistribution of the radiation beam of the third energy level.
 16. Thebeam filter assembly of claim 12, wherein the base filter, the first,second and third filter slices are each independently movable in and/orout of a beamline.
 17. The beam filter assembly of claim 12, furthercomprising a fourth filter slice stackable with the third, second, andfirst filter slices, and/or with the base filter to modify a beamquality of a radiation beam of a fourth energy level higher than thethird energy level.
 18. The beam filter assembly of claim 17, whereinthe fourth filter slice is configured to stack with the third, secondand first filter slices, and the base filter when in use to modify thebeam quality of the radiation beam of the fourth energy level.
 19. Thebeam filter assembly of claim 17, wherein the fourth filter slice isconfigured to stack with the base filter when in use to modify the beamquality of the radiation beam of the fourth energy level.
 20. The beamfilter assembly of claim 17, wherein the fourth filter slice isconfigured to stack with the third, second and first filter slices,and/or with the base filter when in use to modify an energy spectrumand/or a dose distribution of the radiation beam of the fourth energylevel.
 21. The beam filter assembly of claim 17, wherein the basefilter, the first, second, third, and fourth filter slices are eachindependently movable in and/or out of a beamline.
 22. The beam filterassembly of claim 17, wherein the base filter is configured to modify abeam quality of a radiation beam of the base energy level at about 4 MV,the first filter slice is configured to stack with the base filter whenin use to modify a beam quality of a radiation beam of the first energylevel at about 6 MV, the second filter slice is configured to stack withthe first filter slice and the base filter when in use to modify a beamquality of a radiation beam of the second energy level at about 10 MV,the third filter slice is configured to stack with the second and firstfilter slices and the base filter when in use to modify a beam qualityof a radiation beam of the third energy level at about 15 MV, and thefourth filter slice is configured to stack with the third, second, andfirst filter slices and the base filter when in use to modify a beamquality of a radiation beam of the fourth energy level at about 18 MV.23. A beam filter assembly, comprising: a base filter configured tomodify a beam quality of a radiation beam of a base energy level; and Nfilter slices each being stackable with the base filter (N being aninteger of 1-10), wherein the N filter slices are configured such that a1^(st) filter slice is for use at least in modifying a radiation beam ofa 1^(st) energy level, a 2^(nd) filter slice for use at least inmodifying a radiation beam of a 2^(nd) energy level, and in suchsuccessive order that an (N−1)^(th) filter slice is for use at least inmodifying a radiation beam of an (N−1)^(th) energy level, and an N^(th)filter slice for use at least in modifying a radiation beam of an N^(th)energy level, and wherein the N^(th) energy level is higher than the(N−1)^(th) energy level, and in such successive order that the 2^(nd)energy level is higher than the 1^(st) energy level, and the 1^(st)energy level is higher than the base energy level; and wherein when anM^(th) filter slice (M being an integer ≦N) is stacked with the basefilter in use to modify a radiation beam of an M^(th) energy level, the1^(st) through (M−1)^(th) filter slices are stacked between the M^(th)filter slice and the base filter.
 24. The beam filter assembly of claim23, wherein the base filter and the N filter slices are each configuredto modify an energy spectrum of a radiation beam.
 25. The beam filterassembly of claim 23, wherein the base filter and the N filter slicesare each configured to modify a dose distribution of a radiation beam.26. The beam filter assembly of claim 23, wherein the base filter and Nfilter slices are each independently movable in and/or out of abeamline.
 27. The beam filter assembly of claim 26, wherein the basefilter and N filter slices are each independently movable in and/or outof a beamline by a linear motion axis.
 28. The beam filter assembly ofclaim 26, wherein the beam filter and N filter slices are eachindependently movable in and/or out of a beamline by a rotary motionaxis.
 29. A beam filter positioning device comprising a base stagecarrying a base filter and one or more additional stages each carrying afilter slice in and/or out of a beamline, wherein the base stage isprovided with a first engagement site and a second engagement site; theone or more additional stages comprise at least a first stage carrying afirst filter slice and being provided with a first engagement site, asecond engagement site, and an open port, wherein the first stage andthe base stage are each independently movable relative to the beamline;and the first stage is engageable with the base stage when at least oneof the first and second engagement sites of the first stage is alignedwith at least one of the first and second engagement sites of the basestage, and further movable with the base stage in unison when engaged.30. The beam filter positioning deice of claim 29, wherein the firststage and the base stage are each independently movable by a linearmotion axis.
 31. The beam filter positioning device of claim 30, whereinwhen engaged the first stage and the base stage are further movable inunison by an additional linear motion axis.
 32. The beam filterpositioning deice of claim 29, wherein the first stage and the basestage are each independently movable by a rotary motion axis.
 33. Thebeam filter positioning device of claim 32, wherein when engaged thefirst stage and the base stage are further movable in unison by anadditional linear motion axis.
 34. The beam filter positioning device ofclaim 29, wherein the first stage and the base stage are arranged suchthat when the first engagement site of the first stage is aligned withthe first engagement site of the base stage, the first filter slicecarried by the first stage is stacked with the base filter carried bythe base stage, and the first stage is engageable and further movablewith the base stage in unison to position the stacked first filter sliceand base filter in the beamline; or when the second engagement site ofthe first stage is aligned with the first engagement site of the basestage, the open port provided in the first stage is aligned with thebase filter carried by the base stage, and the first stage is engageableand further movable with the base stage in unison to position thealigned open port of the first stage and the base filter carried by thebase stage in the beamline.
 35. The beam filter positioning device ofclaim 34, wherein the first stage and the base stage each furthercarries a scattering foil, and the first stage and the base stage arearranged such that: when the second engagement site of the first stageis aligned with the first engagement site of the base stage, and thefirst movable stage is engageable and further movable with the basestage in unison to position the scattering foil carried by the basestage in the beamline; or when the first engagement site of the firststage is aligned with the second engagement site of the base stage, andthe first stage is engageable and further movable with the base stage inunison to position the scattering foil carried by the first stage in thebeamline.
 36. The beam filter positioning device of claim 29, whereinthe first stage is further provided with apertures at the first andsecond engagement sites of the first stage; the base stage is furtherprovided with apertures at the first and second engagement sites of thebase stage; and the beam filter positioning device further comprises anactuator key operable to engage each of the apertures at the first andsecond engagement sites of the first stage and the base stage.
 37. Thebeam filter positioning device of claim 36, wherein the actuator key iscontrollable by a control.
 38. The beam filter positioning device ofclaim 29, further comprising a second stage carrying a second filterslice, wherein the second stage is provided with a first engagementsite, a second engagement site, and an open port, and independentlymovable relative to the beamline, the second stage is engageable withthe first stage when at least one of the first and second engagementsites of the second stage is aligned with at least one of the first andsecond engagement sites of the first stage, and further movable with thefirst stage in unison when engaged, and the second stage is engageablewith the first stage and the base stage when at least one of the firstand second engagement sites of the second stage is aligned with at leastone of the first and second engagement sites of the first stage and thebase stage, and further movable with the first stage and the base stagein unison when engaged.
 39. The beam filter positioning device of claim38, wherein the first stage, the second stage, and the base stage areeach independently movable by a linear motion axis.
 40. The beam filterpositioning device of claim 39, wherein when engaged the first stage andthe second stage are further movable in unison by an additional linearmotion axis.
 41. The beam filter positioning device of claim 39, whereinwhen engaged the first stage, the second stage and the base stage arefurther movable in unison by an additional linear motion axis.
 42. Thebeam filter positioning device of claim 38, wherein the first stage isprovided with apertures at the first and second engagement sites of thefirst stage; the second stage is provided with apertures at the firstand second engagement sites of the second stage; the base stage isprovided with apertures at the first and second engagement sites of thebase stage; and wherein the beam filter positioning device furthercomprises an actuator key operable to engage each of the apertures atthe first and second engagement sites of the first stage, the secondstage, and the base stage.
 43. The beam filter positioning device ofclaim 42, wherein the actuator key is controllable by a control.
 44. Thebeam filter positioning device of claim 38, wherein the first stage, andsecond stage, and the base stage are arranged such that when the firstengagement sites of the first and second stages are aligned with thefirst engagement site of the base stage, the first filter slice carriedby the first stage and the second filter slice carried by the secondstage are stacked with the base filter carried by the base stage, andthe first and second stages are engageable and further movable with thebase stage in unison to position the stacked first and second filterslices and base filter in the beamline; or when the first engagementsites of the first and second stages are aligned with the secondengagement site of the base stage, the open ports provided in the firstand second stages are aligned with the base filter carried by the basestage, and the first and second stages are engageable and furthermovable with the base stage in unison to position the aligned open portsand base filter in the beamline.
 45. The beam filter positioning deviceof claim 44, wherein the first stage, the second stage, and the basestage each further carries a scattering foil, and the first stage, thesecond stage, and the base stage are arranged such that when the secondengagement sites of the first and second stages are aligned with thefirst engagement site of the base stage, the first and second stages areengageable and further movable with the base stage in unison to positionthe scattering foil carried by the base stage in the beamline; or whenthe first engagement site of the first stage is aligned with the secondengagement sites of the second stage and base stage, and the first stageis engageable and further movable with the second stage and the basestage in unison to position the scattering foil carried by the firststage in the beamline, or when the first engagement site of the secondstage is aligned with the second engagement sites of the first stage andbase stage, and the second stage is engageable and further movable withthe first stage and the base stage in unison to position the scatteringfoil carried by the second stage in the beamline.
 46. The beam filterpositioning device of claim 29, wherein the one or more additionalstages comprise 1-10 stages each carrying a filter slice, provided withan open port, a first engagement site and a second engagement site, andindependently movable relative to the beamline.
 47. The beam filterpositioning device of claim 46, wherein the base stage and the one ormore additional stages each further carries a scattering foil.
 48. Thebeam filter positioning device of claim 46, wherein the base stage andthe one or more additional stages are each independently movable by alinear motion axis.
 49. The beam filter positioning device of claim 46,wherein the base stage is engageable and movable with the one or moreadditional stages in unison by an additional linear motion axis.